Pressure type flow rate control devices are capable of highly accurately controlling flow rates of various types of fluids, such as gas, with a simple mechanism in which a piezoelectric element driving type pressure control valve and an orifice are combined. Many of these pressure type flow rate control devices have been provided for practical use in the field of semiconductor manufacturing equipment, or the like.
Furthermore, because the pressure type flow rate control device performs flow rate control by adjusting gas pressure on the upstream side thereof by use of an orifice by controlling a pressure control valve so as to open and close, it is necessary to constantly monitor so-called “clogging” of the orifice hole. Therefore, a function of self-diagnosing with respect to a level of a change in shape due to clogging, cracks, corrosion, or the like, of the orifice, that is a so-called “flow rate self-diagnosis” (or clogging detection) is provided in some systems.
FIG. 5 shows an example of a conventional pressure type flow rate control device FCS provided with a flow rate self-diagnosis function. That is, the gas pressure and the gas temperature in a pipe passage 3, disposed between a pressure control valve 1 and an orifice 2, are detected by a pressure detector P1 and a temperature detector T1, respectively. The detected gas pressure and gas temperature are input to an arithmetic processing unit (CPU). In the arithmetic processing unit (CPU), a gas flow rate Qc passed through the orifice 2 is computed, and a differential flow rate ΔQ between a setting flow rate Qs and the computed flow rate Qc is computed. A control signal S corresponding to the differential flow rate ΔQ is input to a piezoelectric element driving unit 1a, and the pressure control valve 1 is controlled to be opened and closed in the direction required so that the differential flow rate ΔQ becomes zero.
Then, when the orifice hole diameter of the orifice 2 is changed due to clogging when the pressure type flow rate control device is in use, the pressure drop characteristics in the pipe passage 3 in FIG. 5 are changed. Therefore, the pressure drop characteristics are measured before the pressure type flow rate control device is provided for practical use (i.e., before factory shipment) and are stored as initial values in a memory device M. This makes it possible to determine the presence or absence of malfunction in flow rate control by comparing the initial values with the measured values of the pressure drop characteristics in a diagnostic while that pressure type flow rate control device is in use. That is, the flow rate self-diagnosis is provided in order to self-diagnose the presence or absence of malfunction in flow rate control caused by a change in shape of the orifice hole due to clogging, cracks, corrosion, and the like, of the orifice 2.
In more detail, with reference to FIG. 5, first, before shipment of the pressure type flow rate control device FCS, a gas for flow rate self-diagnosis (usually, an N2 gas) is supplied to the piezoelectric element driving type pressure control valve 1 of the pressure type flow rate control device FCS, and a setting flow rate Qs of the pressure type flow rate control device FCS is set to the flow rate of 100%, and the memory device M is set to an operative state (i.e., setting of an initial value memory signal). It is, as a matter of course, that in the case where the controlling flow rate (i.e., setting flow rate Qs) is less than or equal to a certain threshold value at this time, an alarm AL corresponding to a deficiency in gas supply pressure is given. Next, the piezoelectric element driving type pressure control valve 1 is rapidly completely closed, and the detected pressure, and time data of the pressure detector P1, are measured at predetermined time intervals and are stored in the memory device M (storage of initial value data).
In a flow rate self-diagnosis of the pressure type flow rate control device FCS when in use, first, a gas, which is the same as the gas used for determining the storage of initial value data, is supplied to the piezoelectric element driving type pressure control valve 1 and, at the same time, a setting flow rate Qs thereof is set to the flow rate of 100%. It is, as a matter of course, that in the case where the controlling flow rate (setting flow rate Qs) is less than or equal to a certain threshold value at this time, an alarm AL corresponding to deficiency in gas supply pressure is given. Next, the piezoelectric element driving type pressure control valve 1 is rapidly completely closed, and the detected pressure and time data in the pipe passage 3 at this time are measured at predetermined time intervals, and are compared with the initial value data stored in advance in the memory device M in the arithmetic processing unit (CPU). In the case where a difference between both the detected pressure data and time data is greater than the setting value, an alarm indicating that the diagnosis result is abnormal is given. In FIG. 5, the reference character E designates a power supply voltage.
Meanwhile, a normally-closed type metal diaphragm valve, provided with a piezoelectric element driving unit, is utilized as the piezoelectric element driving type pressure control valve 1 of the pressure type flow rate control device FCS in many cases. A driving voltage is applied to the piezoelectric element of the piezoelectric element driving unit 1a so as to stretch its entire length, thereby lifting up a valve stem against an elastic force of an elastic body, that opens the valve. Furthermore, when the voltage applied to the piezoelectric element is eliminated, the length of the piezoelectric element is restored to its initial length, and the valve stem is lowered by the elastic force of the elastic body, thereby closing the valve (see, e.g., Japanese Published Unexamined Patent Application No. 2005-149075).
As a result, variation occurs in time from when the valve is fully opened until when the valve is completely closed by necessity according to a speed at which the voltage applied to the piezoelectric element is eliminated (hereinafter, called “delay in dropping of a piezoelectric element driving voltage”). In addition, a time position at the first sampling point in measurement of the pressure drop characteristics (i.e., sampling start time) fluctuates, which makes it difficult to precisely measure the pressure drop characteristics. In addition, the greater the amount of displacement in stroke of the piezoelectric element, the greater the piezoelectric element driving voltage, and the greater the outer diameter of the valve disc, then the greater is the influence on the pressure drop characteristics caused by the delay in dropping of the piezoelectric element driving voltage. Furthermore, the shorter the sampling time for a diagnostic, the greater is the influence on the pressure drop characteristics caused by the delay in dropping of the piezoelectric element driving voltage.
FIG. 6 shows an example of pressure drop characteristics of the conventional pressure type flow rate control device (FCS type: manufactured by Fujikin Incorporated). It is clear from FIG. 6 that, as the gas supply pressure (kPaG) is lowered, even the pressure drop characteristics curve of the same pressure type flow rate control device moves upward.
Furthermore, FIG. 7 shows research data of the influence on the diagnosis results (%) caused by a change in pressure drop characteristics due to a fluctuation in gas supply pressure. Generally, in the normally-closed type piezoelectric element driving type pressure control valve 1, when the supply pressure is low, the piezoelectric element voltage becomes higher than in the case where the supply pressure is high, which makes it easy for the pressure control valve closing time delay to occur (Japanese Published Unexamined Patent Application No. 2005-149075). As a result, when the supply pressure is lowered, the piezoelectric element driving voltage rises to increase the pressure control valve closing time delay, and the diagnosis result (%) fluctuates toward the positive side as shown in FIG. 7.
FIG. 8 is a block diagram showing the configuration of a piezoelectric element driving circuit of the conventional piezoelectric element driving type pressure control valve. A driving voltage is supplied to the piezoelectric element (capacitance C) from the power supply through a field-effect transistor FET1, an inductor L, and a diode D from the arithmetic processing unit (CPU) of the pressure type flow rate control device FCS. In more detail, when a field-effect transistor FET2 is turned on by a step-up command signal from the CPU, a voltage is induced in the inductor L and, thereafter, when the field-effect transistor FET2 is turned off, the induced voltage in the inductor L is superimposed on the driving voltage. Then, the voltage on which the induced voltage is superimposed is applied (charged) as a piezoelectric element driving voltage to the piezoelectric element (i.e., modeled as a capacitor). Furthermore, in the case wherein pressure rising continues, the field-effect transistor FET2 is repeatedly turned on and off. Furthermore, the entire length of the piezoelectric element stretches by charging the piezoelectric element, which opens the pressure control valve.
On the other hand, in the case when the pressure control valve is completely closed, a step-down field-effect transistor FET3 is turned on by input of a step-down command signal from the CPU to discharge the charged voltage of the piezoelectric element (i.e., modeled as a capacitor). Consequently, the piezoelectric element contracts, and the pressure control valve is closed by a spring elastic force.
Patent Document 1: Japanese Published Unexamined Patent Application No. H8-338546
Patent Document 2: Japanese Published Unexamined Patent Application No. 2000-137528
Patent Document 3: Japanese Published Unexamined Patent Application No. 2005-149075